This application is related to application Ser. No. 08/509,586 entitled A Method of Estimating Signal Quality for a Direct Sequence Spread Spectrum Receiver; application Ser. No. 08/509,588 now U.S. Pat. No. 5,654,911 entitled Fast Acquisition Bit Timing Loop Method and Apparatus; application Ser. No. 08/509,590 entitled Short Burst Direct Acquisition Direct Sequence Spread Spectrum Receiver; application Ser. No. 08/509,589 now U.S. Pat. No. 5,675,339 entitled A/D Reference Level Adjustment Circuit to Maintain an Optimum Dynamic Range at the A/D all of which are incorporated by reference, and are filed concurrently herewith and assigned to the assignee hereof.
The present invention relates digital data radio receivers, and more particularly to a digital data radio receiver with diverse antennas and method in which a data signal preamble is rapidly evaluated to determine which of the antennas is best used to receive the subsequent data signal.
A network of digital data radios may operate as a loosely coupled network of nodes in which each receiver is required to rapidly acquire data transmissions without prior knowledge of the time of transmission, frequency offset of the transmission (from a known frequency), or the transmission mode.
A typical data transmission includes a data signal preceded by a preamble that typically consists of a scrambled all-ones pattern, followed by a unique word. The radio receiver is expected to acquire the preamble before attempting to demodulate the data signal that follows. The probability of detecting and acquiring the preamble is desirably high and the false alarm rate (declaring acquisition on noise) is desirably low, even in a noisy environment. Further, the receiver desirably is sufficiently robust to accommodate carrier frequency offsets that can create phase rotations of 45.degree. per symbol (the term symbol is used herein to refer to the units used in the preamble format, e.g., spread spectrum BPSK), and to accommodate rapid changes in noise level.
Receivers in such systems may have a single antenna, or several diverse antennas to improve reception. Receivers connected to multiple antennas are expected to identify the one of the multiple antennas that is to receive the data signal that follows the preamble, and because the preamble has a set length the receiver must attempt to acquire the preamble on all of the antennas and identity the best antenna within the limited time period of the preamble.
Prior art receivers are typically too slow to operate effectively with multiple antennas, and even with only one antenna do not operate with high probability of detection and low false alarm rate in a noisy environment. For example, typical receivers use a symbol length matched filter with the output acquired by a phase locked loop to remove the offset frequency of the carrier. Acquisition is declared based on the amplitude of the correlation peaks from the matched filter. The disadvantage of this approach is that the phase locked loop is slow and may have large amounts of the jitter if the signal level is near the noise level. To avoid this problem in multiple antenna systems, it is known to provide parallel processing paths for the antennas, with the attendant added cost and complexity of separate phase locked loop circuits for each processing path.
Accordingly, it is an object of the present invention to provide a novel digital data radio receiver and method that is suitable for operation with one or multiple antennas and is able maintain an acceptable probability of detection and false alarm rate, while avoiding the problems of the prior art.
It is another object of the present invention to provide a novel digital data radio receiver and method in which the variance of frequency offsets for symbols in a block of symbols in the preamble and the average magnitude of the symbols in the block of symbols are determined, and in which these values are used together to evaluate the presence and quality of a data transmission.
It is still another object of the present invention to provide a novel digital data radio receiver and method in which the variance of frequency offsets and average magnitude of symbols in several blocks of symbols in the preamble are evaluated to improve the accuracy of the evaluation of whether a detected signal is noise or a data signal preamble.
It is yet another object of the present invention to provide a novel digital data radio receiver with antenna diversity and method in which blocks of symbols from a data signal preamble are provided from alternating antennas serially to the receiver for processing and determination whether a signal is present and which antenna has the higher quality reception.
It is a further object of the present invention to provide a novel digital data radio receiver and method in which the receiver has a demodulator with correlators for providing output signals and in which the correlators sample each symbol in a block of symbols in the preamble at twice a predetermined chip rate and store each of the samples, while using every other stored sample to determine correlator output signals.
It is yet a further object of the present invention to provide a novel demodulator for a digital data radio receiver in which outputs from correlators for I and Q components are combined and converted to polar coordinates, and in which the angle and magnitude in the polar coordinate signal are used to determine the variance of the frequency offset and the signal level that are in turn used to evaluate signal quality.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.